The invention relates to an exhaust gas conduit of an internal combustion engine with a starting catalyser which is arranged near the engine, having a casing with a central passage surrounded by a catalyser body.
A generic exhaust gas conduit is disclosed in EP-A 0 417 412. After the main catalyser starts, the catalyser body of the starting catalyser is still subjected to a partial flow of hot exhaust gas, directly and due to thermal contact with the hot central duct wall, when the exhaust gas butterfly valve is open. This leads to overheating, and finally to premature aging of the catalyser body.
An object of the invention is an exhaust gas conduit wherein overheating of the starting catalyser is prevented after the main catalyser starts.
According to the present invention, a pipe section of the exhaust gas conduit starting from an inlet flow end casing section protrudes into the central passage so as to form an annular passage ends at an upstream distance from the exhaust gas butterfly valve.
Because of the configuration of the annular passage, the catalyser body is completely separated from the exhaust gas when the exhaust gas butterfly valve is open. This prevents the catalyser body from being subjected to the hot exhaust gas (with the associated overheating) after the main catalyser has started.
In an advantageous embodiment of the invention, the annular passage formed by the pipe section of the exhaust gas conduit extends at least over the length of the catalyser body. By this means, thermal contact between the catalyser body and the exhaust gas flow via the central passage wall is prevented, with little manufacturing complication, when the exhaust gas butterfly valve is open. Excessively strong heating of the catalyser body is therefore avoided.
In the cold starting phase with the exhaust gas butterfly valve closed, furthermore, the fact that the exhaust gas is first guided through the annular passage along the complete length of the catalyser body before it acts on the latter has an advantageous effect. The result is accelerated heating of the starting catalyser which therefore begins functioning earlier. The pollutant emission during a cold start is still further reduced by early operation of the starting catalyser.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.